Patches of Maryland's shimmering green forests are turning prematurely brown this summer due to a population explosion of an industrious beetle called the locust leaf miner.

The black-and-orange insects have infested black locust trees from the Alleghenies to the Chesapeake Bay in what may be the worst outbreak in five years, plant and bug scientists say. But no one seems particularly concerned.

Leaf miners, it seems, are perennial pests that seldom do any lasting damage to the hardy, fast-growing black locust, a tree with no commercial value that grows wild throughout the Eastern United States.

Occasionally, the insects also attack apple, birch, beech, cherry, elm, oak and hawthorn. Even then, they are seldom lethal.

"There may be some rare exception where a tree does get [infested] so bad that it dies, but I've never seen it in 18 years of looking," said Ray Bosmans, a plant specialist with the University of Maryland Cooperative Extension Service.

Black locusts are among the first trees to spring up in newly cleared ground, and they proliferate on the shoulders of highways and the edges of fields, he said.

Mary Kay Malinoski, an entomologist with the extension service, said that a black locust could succumb if, as rarely happens, the bugs kill all its leaves during several growing seasons in a row. The trees generally grow two sets of leaves every year.

The locust leaf miner beetle, which is about the size of a pencil eraser, gets its name because its larvae burrow into the black locust leaf and chew out caves and caverns.

As the larvae expand their mine, the hollow leaf dies. All that's left is a brittle shell that often looks as if it's been scorched or burned.

"When people see this, they think it's air pollution, or the heat is turning the leaves brown," Mr. Bosmans said. "It isn't until you get up to the leaf that you see the insect eating."

The destruction will end soon.

The beetles are growing out of their larval stage, when they do the most damage, and the black locusts will soon sprout their second set of leaves.

"People looking at trees in the next couple of weeks will see new flush of growth at ends of branches," Mr. Bosmans said.

While inspecting a tomato field in Newburg recently, Pam King, the Charles County extension agent, was surprised by how much of the surrounding woods had turned the color of toast. "You could see the locust trees were just totally brown against the green of the oaks," she said. "They always brown out, but this year there are some that I don't see any green on."

Sandy Scott, a horticulture consultant with the extension service in Washington County, said, "As you drive down Route 70 from Middletown to Hagerstown, the leaves on the locust trees are all brown."

Ms. Scott said she has counseled a number of homeowners new to the area. "All of a sudden their trees turn brown, and they think something's wrong with them," she said. She recommends patience, not pesticides. "We really don't like to see people using chemicals unless they really have to," she said.

Motorists have also noticed concentrations of the blighted trees along Interstate 70 and U.S. 29 in Howard County, Route 140 in Carroll County and I-95 between Baltimore and Washington.

Some blame this year's outbreak of the leaf-chomping beetles on the temperature this past winter, which was the mildest in the 97 years that the federal government has kept records.